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Ch. 8 - Chromosome Mutations: Variation in Number and Arrangement
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 8 - Chromosome Mutations: Variation in Number and ArrangementProblem 18
Chapter 8, Problem 18

Mendelian ratios are modified in crosses involving autotetraploids. Assume that one plant expresses the dominant trait green seeds and is homozygous (WWWW). This plant is crossed to one with white seeds that is also homozygous (wwww). If only one dominant allele is sufficient to produce green seeds, predict the F₁ and F₂ results of such a cross. Assume that synapsis between chromosome pairs is random during meiosis.

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Step 1: Understand the genetic setup. We have an autotetraploid organism with four copies of each gene. The green seed trait is dominant (W), and white seed is recessive (w). The homozygous green parent genotype is WWWW, and the homozygous white parent genotype is wwww.
Step 2: Determine the genotype of the F₁ generation. Since the green parent (WWWW) is crossed with the white parent (wwww), all F₁ offspring will inherit two W alleles from the green parent and two w alleles from the white parent, resulting in the genotype WWww.
Step 3: Understand allele segregation in autotetraploids. During meiosis, chromosome pairing (synapsis) is random, so gametes can have different combinations of W and w alleles. The possible gametes from a WWww individual can be calculated using the binomial distribution for allele segregation.
Step 4: Calculate the gamete genotype frequencies from the F₁ (WWww). The possible gametes are WW, Ww, and ww, with probabilities given by the formula for combinations of alleles in autotetraploid meiosis: \(P(k) = \frac{\binom{2}{k} \binom{2}{2-k}}{\binom{4}{2}}\), where \(k\) is the number of W alleles in the gamete.
Step 5: Predict the F₂ generation genotypes and phenotypes by crossing the gametes from the F₁ generation. Use the gamete frequencies to set up a Punnett square or probability matrix, then determine the genotype frequencies of the F₂. Since only one dominant allele is needed for green seeds, calculate the proportion of green (at least one W) and white (all w) phenotypes.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Autotetraploidy and Chromosome Pairing

Autotetraploids have four copies of each chromosome, originating from genome duplication within a species. During meiosis, chromosomes can pair randomly as bivalents or multivalents, affecting segregation patterns. This random synapsis alters expected Mendelian ratios compared to diploids, influencing genotype and phenotype frequencies in offspring.
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Dominance and Allelic Interaction in Polyploids

In polyploids, a single dominant allele can mask the effect of multiple recessive alleles, producing the dominant phenotype. For a trait controlled by one dominant allele (e.g., green seeds), any genotype with at least one 'W' allele will show green seeds, while only the homozygous recessive (wwww) shows white seeds. This affects phenotypic ratios in progeny.
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Segregation and Phenotypic Ratios in Polyploid Crosses

Segregation in autotetraploids follows more complex patterns due to multiple chromosome copies and random pairing. The F₁ generation from a homozygous dominant (WWWW) crossed with homozygous recessive (wwww) will be all heterozygous (Wwww) and green. The F₂ generation shows modified ratios, with a higher proportion of dominant phenotypes than diploid Mendelian ratios due to multiple allele combinations.
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Related Practice
Textbook Question

Drosophila may be monosomic for chromosome 4, yet remain fertile. Contrast the F₁ and F₂ results of the following crosses involving the recessive chromosome 4 trait, bent bristles:

monosomic IV, normal bristles x diploid, bent bristles.

Textbook Question

The outcome of a single crossover between nonsister chromatids in the inversion loop of an inversion heterozygote varies depending on whether the inversion is of the paracentric or pericentric type. What differences are expected?

Textbook Question

Certain varieties of chrysanthemums contain 18, 36, 54, 72, and 90 chromosomes; all are multiples of a basic set of nine chromosomes. How would you describe these varieties genetically? What feature do the karyotypes of each variety share? A variety with 27 chromosomes has been discovered, but it is sterile. Why?

Textbook Question
The mutations called bobbed in Drosophila result from variable reductions (deletions) in the number of amplified genes coding for rRNA. Researchers trying to maintain bobbed stocks have often documented their tendency to revert to wild type in successive generations. Propose a mechanism based on meiotic recombination which could account for this reversion phenomenon. Why would wild-type flies become more prevalent in Drosophila cultures?
Textbook Question

Drosophila may be monosomic for chromosome 4, yet remain fertile. Contrast the F₁ and F₂ results of the following crosses involving the recessive chromosome 4 trait, bent bristles:

monosomic IV, bent bristles x diploid, normal bristles

Textbook Question

Having correctly established the F₂ ratio in Problem 18, predict the F₂ ratio of a 'dihybrid' cross involving two independently assorting characteristics (e.g., P₁ = WWWWAAAA x wwwwaaaa).